1. Field of the Invention
The invention relates to solid polyurethane hardeners (curing agents) having triazine groups, to processes for preparing such compounds and to their use for preparing plastics, especially powder coating materials which crosslink to give high-gloss or matt, light- and weather-stable coating films.
2. Discussion of the Background
Externally or internally blocked polyisocyanates which are solid at room temperature constitute useful crosslinkers for thermally crosslinkable polyurethane (PU) powder coating materials. For example, DE-C 27 35 497 describes PU powder coating materials having outstanding weathering and thermal stability. The crosslinkers whose preparation is described in DE-C 27 12 931 consist of E-caprolactam-blocked, isocyanurate-functional isophorone diisocyanate. Urethane-, biuret- or urea-functional polyisocyanates whose isocyanate groups are likewise blocked are also known.
The disadvantage of these purely externally blocked systems lies in the stoichiometric elimination of the blocking agent during the thermal crosslinking reaction. Since, the blocking agent is able to escape into the environment, it is necessary on ecological and workplace safety grounds to take special measures to clean the waste air and/or to recover the blocking agent. Moreover, the crosslinkers are of low reactivity, and curing temperatures of more than 170.degree. C. are required.
DE-A 3030539 and DE-A 3030572 describe processes for preparing uretdione-functional polyaddition compounds whose terminal isocyanate groups are blocked irreversibly with monoalcohols or monoamines. The chain-terminating constituents of the crosslinkers are particularly disadvantageous, leading to low network densities of the PU powder coatings and thus to moderate solvent resistances.
Hydroxyl-terminated, uretdione-functional polyaddition compounds are the subject of EP 0669353. Because of their di-functionality they have improved solvent resistance. A common feature of the powder coating materials based on these uretdione-functional polyisocyanates is that they do not emit any volatile compounds in the course of the curing reaction. However, the stoving temperatures are high, at temperatures of 180.degree. C. or more.
The use of amidines as catalysts in PU coating materials is described in EP 803 524. However, there is no chemical reaction of the PU hardener with the catalyst.
It is therefore an object of the present invention to provide low-emission PU hardeners of high reactivity which are particularly suitable for producing plastics and ecologically valuable powder coating materials.